If you tell us what you like to read and we can better help you.
I prefer books without DRM. I also like books from "new" writers. I also like old stories. I also like....
But here goes:
http://www.baen.com/library/
http://www.ebookwise.com/
http://www.fictionwise.com/
http://gutenberg.net.au/
http://www.webscription.net/
and ofcourse
https://www.mobileread.com/forums/forumdisplay.php?f=24
for more suggestions.